1. Field of the Invention
The present invention relates to a digital signal processing apparatus and, more particularly, to a digital signal processing apparatus for suppressing aliasing noise which is caused in association with half-band processing of a digital filter.
2. Description of the Background Art
In accordance with the advancement of digital technologies, various digital signal processing technologies are proposed in wide fields. In the above-mentioned digital signal processing, an analog input signal is converted into a digital signal by an analog/digital (hereinafter, referred to as an A/D) converter and then is subjected to various signal processing by latter-stage circuits. A digital filter serving as a general digital signal processing circuit performs calculation for a plurality of sampling data which is continuously inputted, attenuates a frequency component other than a normal band from among frequency components included in sampling outputs, and extracts only desired frequency component data. In general, the digital filter uses a method for increasing the amount of sampling data so as to improve an signal to noise (S/N) ratio. However, increasing the amount of sampling data causes an increase in circuit scale and in the amount of calculation times thereby prolonging calculation time.
Generally, half-band processing is well-known to solve the problem. In half-band processing, the ability to reduce the number of elements is increased as the amount of sampling data is larger, and calculation processing is facilitated by using the principle in that the S/N ratio of an output value is not changed upon setting a coefficient, which is multiplied by even-th sampling data, to be zero when the number of sampling data serving as a first processing target is odd. Since the coefficient is zero, the amount of calculation times is half. As a consequence, the number of multipliers, adders, and complement control circuits and the bit length of each register can be reduced.
However, since processing for the even-th sampling data is not performed in the half-band processing of the above-mentioned digital filter, it is equivalent that a sampling rate is reduced to ½. Then an aliasing-noise pass band when reducing the sampling rate is caused.
FIG. 1 is a diagram showing pass band characteristics of an output signal from the digital filter, in which the aliasing-noise is caused by the half-band processing, according to background art. An aliasing band 1, occurring because of the half-band processing, occurs at almost a half frequency of a sampling frequency of the digital filter, as a pass band different from a normal band 2 serving as an original pass band. Therefore, the analog input signal includes a frequency component having the above band and, when the frequency component has an influence on signal processing at the later stage of the digital filter, the half-band processing cannot be used. Even when purposely using half-band processing, a digital low-pass filter for anti-aliasing is necessary at the later stage of the digital filter. However, the digital low-pass filter needs calculation processing for a digital filter output and, therefore, the circuit scale and the number of calculation times are increased and the calculation time is long. An advantage for using the half-band processing to the digital filter is lost.